Tick " Allow Guest To Access My Local Network" on guest network setting screen.

DHCP should be disabled when working as access point. From what you described, it may not work if WR940N works as wireless access point. When your pc connects to the wall port, do you need to set a...

http://www.tp-link.com/en/faq-699.html

You can reset 940N to config it again. Or move it closer. Remember to change Lan IP address and turn off DHCP on 940N.

I am not sure if it can control the car engine heater or not, you'd better check the spec. and also the temperature can't be very low for a electric appliance

Did you unzip the file? BTW, why do you need to upgrade the firmware?